Hybrid pneumatic-magnetic isolator-actuator

ABSTRACT

The invention disclosed is a compact and lightweight hybrid pneumatic-magnetic isolator-actuator capable of large force, substantial stroke and bandwidth actuation with near frictionless operation and vibration isolation with very low break frequency. Pneumatic and magnetic forces are applied to a single carriage comprised primarily of a coaxially arranged air piston and coil. The carriage is driven relative to a frame or housing including an internally mounted cylindrical piston sleeve and magnetic actuator body. A combination of air bearings and air bearing piston construction provide for frictionless motion of the carriage relative to the frame. The pneumatic piston provides the actuation force for both static loads and low frequency dynamic loads. An integrally mounted sensor and control unit determine the pressure error resulting at the pneumatic piston. The control unit utilizes the pressure error to drive a high bandwidth magnetic actuation capability in parallel with the pneumatic actuation capability. An air tank of prescribed volume may be connected to the pneumatic piston for effecting a desired air-spring stiffness upon the isolator-actuator.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of pending U.S. patent application Ser.No. 10/078,320, filed Feb. 20, 2002.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with U.S. Government support under a secondlevel subcontract to Contract No. F29601-97-C-0001 awarded by theDepartment of the Air Force. The U.S. Government has certainroyalty-free rights in this invention.

BACKGROUND OF THE INVENTION

The present invention relates to vibration isolators and positioncontrol actuators. More particularly, the present invention pertains toa compact hybrid pneumatic-magnetic isolator-actuator capable of largestroke, near frictionless operation, and high ratio of actuation forceto weight.

Passive isolation systems, composed of multiple passive vibrationisolators, are commonly used for preventing vibration force input to apayload from a vibrating support base. Base motions of a frequencysufficiently higher than the break frequency of the isolation system(i.e. the natural frequency of the payload on the isolators) aresignificantly attenuated by the isolators and are prevented from passinginto and disturbing the payload. Soft mounts (passive isolators) simplyallow the payload to be held still in inertial space by its own inertia,but only at frequencies high enough that inertia forces on the payloadare large compared to stiffness forces transmitted through theisolators, i.e. at frequencies above the system natural frequencies.

The static load position of the payload and the movement of the payloadunder low frequency dynamic loads, i.e. those loads having vibrationfrequency components below the break frequency, are dependent upon thespring stiffness of the isolators, the mass of the payload, and the lowfrequency acceleration forces being applied to the payload. When thebody acceleration forces are constant, payload position relative to thebase remains constant. When low frequency body acceleration forces upona payload vary, such as would the forces on a payload in an aircraftundergoing various maneuvers, the payload position relative to the basevaries. This is often detrimental to the function of the payload.

The nature of passive isolators is that they do not provide for force orposition control that is necessary for payload stabilization whenvarying low frequency body forces are applied. Rather the force appliedto the payload by a passive isolator is dependent upon the displacementof the payload relative to the support base. Passive isolation thereforealways involves a compromise between position control accuracy anddynamic isolation.

To control the position of the payload relative to its base and maintainvibration isolation, an active isolator, i.e. an isolator-actuator isrequired. The isolator-actuator actively controls the low-frequencyforce applied to the payload at any and all positions of the payloadrelative to the base, i.e. regardless of the extension position of thesupport isolators.

Position actuators such as hydraulic actuators are commonly used forposition control. Hydraulics can afford significantly high bandwidthposition control because of the inherent incompressibility of thehydraulic fluid. However hydraulic actuators have several disadvantages.Position control and base motion isolation can be effected only up tothe bandwidth limit. At the bandwidth limit, often set by dynamics ofthe payload, the actuators become stiff and unable to provide isolationto base motions of higher frequency. Secondly, high fluid pressures,which allow wide bandwidth performance, require tight sliding seals toprevent leakage. These seals inevitably introduce friction, whichdegrades high frequency isolation and introduces a deadband into lowfrequency position control. In effect, seal friction produces dynamicforces on the payload, thereby defeating the desired isolation function.Finally, hydraulic actuators are undesirable for use in vacuum becauseeven very small oil leakage would cause contamination of any nearbyequipment.

Pneumatic isolators with air bearing support have been used forproviding low to zero friction payload support. However, a dilemma facedin the use of pneumatic isolators as actuators is that thecompressibility of the air in the isolator-actuator and pneumaticcontrol valve severely limits the control bandwidth.

Additional control problems arise from the control system componentsnecessary to drive the actuators. Servo-valves used for either hydraulicor pneumatic control are commonly proportional spool valves, whichalways have a degree of friction and therefore produce hysteresis in theactuator control.

Many of the problems described above have been addressed in an earlierpatent, U.S. Pat. No. 6,196,514 B1, by this inventor, which disclosed aLarge Airborne Stabilization/Vibration Isolation System (AS/VIS). Inthat patent a payload is supported by an array of frictionless pneumaticisolators integrated with large, high-force electromagnetic voice coilactuators. The pneumatic isolators support the entire payload weightwith very low stiffness and no friction. The voice coil actuators act inparallel with the isolators to effect position control with reasonablyhigh bandwidth without degrading high-frequency isolation.

However the AS/VIS design has several limitations. The voice coilactuators are quite heavy, being required to control the position of alarge payload. The passive elements connecting the payload to ground invertical and horizontal directions are “nested”, meaning one rests uponthe other and they act in series. While producing excellent isolation,the system is too fragile to meet the crash load requirements for anairborne system, and thus requires a parallel system of slack tethers inorder to meet airworthiness requirements. Finally, the uncontrolledexhaust from the air bearings of the isolator-actuators render thesystem unsuitable for use in vacuum. The present invention advances thestate of the art by addressing all three of these limitations.

Other devices of the prior art have offered damping and vibrationisolation with active control of the isolator damping characteristics,but they do not provide both actuation with high force capacity and theisolation characteristics of very soft passive mounts. U.S. Pat. No.6,003,849 discloses a hybrid isolator and structural control actuatorstrut. In this device communicating fluid reservoirs are used to providepayload motion damping and vibration isolation, and an activeenhancement mechanism is used to alter the fluid pressure in the fluidreservoirs to modify the damping characteristics and add modestactuation capability to the device. This device fails to provide thelarge actuation force and stroke capability and low break frequencyisolation needed for stabilization of large airborne optical systems.U.S. Pat. No. 6,129,185 similarly offers vibration damping and isolationwhile magnetically destiffening the support system to offer lowerisolation frequencies, but it does not afford the combined actuationability and high degree of isolation of the present invention.

Actuation capability is afforded in other devices of the prior art butwithout the high degree of isolation, and near frictionless operation ofthe present invention. U.S. Pat. No. 5,060,959 describes an electricallypowered active suspension strut for a vehicle. This device incorporatesa spring or fluid system for load support and therein fails to providethe very low frequency vibration isolation and near frictionlessactuation needed. Additionally, actuation capability is limited to theforce capability of the electric motor.

There remains a need for a compact, lightweight, high bandwidth actuatorhaving essentially frictionless performance and relatively large strokeand force capacity while also having the characteristics at highfrequency of a very soft passive. The invention described herein is sucha device.

BRIEF SUMMARY OF THE INVENTION

The invention disclosed is a compact and lightweight hybridpneumatic-magnetic isolator-actuator capable of large force, substantialstroke and high bandwidth actuation with near frictionless operation andwhich behaves at high frequency like a very soft passive isolationmount.

The invention obtains superior low-frequency position control andsimultaneous, superior base motion isolation. The invention controls thelow-frequency forces applied to the payload according to auser-specified control law and sensor inputs, rather than simply inpassive response to payload position. High frequency forces are stilldetermined by base-relative position but are kept to low levels, similarto very soft passive isolators. The invention also allows formaintaining position control relative to an arbitrary reference, whichmay itself be moving in inertial space, rather than only relative to thebase on which the payload is supported.

The invention combines pneumatic and magnetic forces on a single movingcarriage assembly, comprised of a coaxially arranged piston and lowerair bearing, journal shaft, coil and coil carrier structure. Thiscarriage moves on air bearings relative to a frame or housing. The frameconsists of a cylindrical sleeve surrounding the piston, a magnet bodysurrounding the coil, an upper air bearing surrounding the journalshaft, and a main housing supporting the cylindrical sleeve, magnetbody, and upper air bearing. The outer diameter surface of thecylindrical sleeve serves as the journal for the lower air bearing. Theconstruction of the invention thus allows for parallel pneumatic andmagnetic forces to be developed between the carriage and frame in theform of a relatively lightweight and compact uniaxial strut. The frameof the strut attaches by a ball joint or universal joint to thesupporting base and the carriage attaches by a similar ball joint oruniversal joint to the supported payload. The struts may be used in setsof six or more to completely support a payload in all degrees offreedom.

An important aspect of the invention when constructed in the form of auniaxial strut is that bending loads are avoided. Substantially higherloads can be carried than that carried by those systems of the prior artwhich rely on the nesting of separate isolators and actuators.

The pneumatic piston provides static and low frequency actuation forcefor supporting the payload weight and for maintaining a desired payloadposition against low-frequency disturbances, such as inertial loads onthe payload caused by aircraft maneuvering. Gas pressure, such as fromair, nitrogen, or other gas, applied to the piston is controlled by apressure servo. The pressure servo is comprised of a pneumaticservo-valve, a suitable drive amplifier and compensation for the valve,and a pressure transducer, all arranged in a feedback loop. The pressureservo allows the pressure on the piston, and thus the piston force, tobe controlled by an input command signal. The invention therebyincorporates the capabilities of a pneumatic actuator with proportionalcontrol of its output force.

In a preferred embodiment, the piston force command may be derived fromtransducers sensing the payload position, either relative to thesupporting base or to some other reference which may itself be movingrelative to the base. Thus the pneumatic actuator capability becomespart of the position control system for the payload. The bandwidth ofthis position control system is small compared to the lowest flexuralnatural frequency of the payload, both to ensure control systemstability and to avoid exciting resonances of the payload. Above thepressure servo bandwidth, the strut behaves like a conventionalpneumatic spring, with stiffness determined by piston area and total gasvolume. This spring can be quite soft for good low through highfrequency isolation since, like any pneumatic spring, its stiffness isnot related to vertical deflection, or sag, under the payload weight.Like any pneumatic spring, the static sag due to payload weight can beadjusted to zero by controlling the gas pressure inside the spring.Furthermore, since the actuation is effected by venting air in and outof the cylinder, the actuator is not required to waste part of its forcecapacity in overcoming the passive stiffness force of a parallel spring,as is the case in active destiffening schemes. In effect, the pneumaticactuator and pneumatic spring are one and the same.

The pneumatic actuator capability, by itself as described above, remainslimited in bandwidth by compressibility of the gas medium and is subjectto hysteresis due to friction in the spool mechanism of the pneumaticservo-valve. Both problems are addressed and largely overcome by amagnetic actuator capability operating in parallel to the pneumaticactuator capability. The magnetic actuator capability is effected by acurrent supplied to a coil surrounding the magnetic actuator body thatis controlled to be proportional to the instantaneous error in thepressure servo, i.e. the difference between the commanded pressure andthe actual pressure. The proportionality constant is chosen such thatthe magnetic force exactly makes up for this difference, and thuscorrects the error. This effective magnetic actuator subsystem comprisedwithin the invention is capable of doing this because of its inherentlywider bandwidth and linearity.

The present invention, effecting a hybrid pneumatic-magnetic actuator,achieves essentially the dynamic behavior (i.e. frequency response) ofthe magnetic actuator alone, however it is much lighter than anall-magnetic actuator of the same force capacity. This is because thelow frequency components of the output force are produced by thepneumatic actuator portion which has an inherently greater ratio ofactuation force to weight.

The invention is particularly well suited to the problem of stabilizingand vibration-isolating a sensitive payload within a large aircraft. Inthat environment, much of the disturbances which the stabilizing systemmust counteract are at low frequency, being due to aircraft maneuveringand low-order flexural modes of the airframe. A large part of therequired actuation force can thus be produced by the relativelylightweight pneumatic subsystem. The magnetic subsystem, with its lowerthrust/weight ratio, can be fairly small since it need only supply thesmaller, higher frequency part of the force spectrum, plus correctingfor error due to phase loss and hysteresis in the pneumatics.

The magnetic force is produced by a coil attached and mounted coaxiallyto the piston and journal shaft by a coil carrier structure. The magnetbody of the voice coil actuator is mounted within the housing coaxialwith the journal shaft. The coil carrier structure passes through oneend of the magnetic actuator body and supports the coil within theenvelope of the magnetic actuator body. Magnetic forces developedbetween the coil and magnetic actuator body are transferred from thecoil through the coil carrier structure and into the journal shaft thusadding the magnetic force to the pneumatic force applied via the pistonto the journal shaft.

Furthermore, the isolator-actuator disclosed provides for nearfrictionless actuation and motion of the actuator carriage through thesupport of the carriage entirely on an air or gas film within the frame.In a preferred embodiment, the invention provides for the piston forceto be exactly proportional to piston pressure, i.e. there being nofriction drag between the piston and cylinder. Such behavior is ensuredby the design of the frictionless piston. The frictionless piston isessential in order to use the magnetic actuator to correct for error inthe pneumatic actuator. Frictionless behavior allows the instantaneouspneumatic force to be exactly measured by measuring the instantaneousgas pressure. Any friction would produce a difference between the truepneumatic force and that inferred from the product of measured pressuretimes known piston area.

In this embodiment the carriage is comprised of a pneumatic piston thatis supported and guided within a cylindrical sleeve fixed to the frame.Surrounding the sleeve is a cylindrical lower air bearing. The lower airbearing and piston are coaxial and are both joined to a flange on thelower end of the journal shaft such that they all move together as partof the carriage. The bearing is held in a housing rigidly attached tothe flange. The piston is attached via a connecting rod with sphericaljoints at either end. These joints allow the piston to align itself withthe cylinder in spite of small dimensional errors in fabrication of thecarriage and frame. Small metering orifices through the piston skirtproduce an air bearing film around the skirt and cause the piston to beself-centering within the cylinder. This piston moves axially within thecylinder without friction because the self-centering action of the airbearing film prevents the piston from touching the cylinder. The airbearing film is supplied from the pressurized air beneath the piston,which also produces the axial force on the piston. The piston outersurface is guided on the air film along the sleeve's inner cylindricalsurface, and the lower air bearing inner cylindrical surface is guidedon an air film by the sleeve's outer cylindrical surface. The upper endof the carriage assembly is likewise supported by a journal air bearing.The bearing is supported on the strut frame and an air film is developedbetween the inner diameter of the bearing and the outer diameter of thejournal shaft. Both upper and lower air bearings are pressurized withcompressed air from an external source. The motion of the carriage isthereby constrained to move along the longitudinal axis of the strut anddoes so supported completely on a film of air. The only friction is thatwhich may be produced by the rubbing of the small internal air linessupplying the bearings and wires supplying current to the magneticactuator coil against the inner surfaces of the strut main housing.

In the embodiment the piston is coaxially connected to a journal shaftvia a common connecting plate, or flange plate, such that the journalshaft transfers the piston pressure force to the payload. The journalshaft, being part of the carriage, is also laterally supported by anupper air bearing fixed within the frame such as to provide alignmentand frictionless longitudinal motion of the carriage within the frame ofthe strut. In a preferred embodiment the frame is further comprised of ahousing which serves to contain the gases exhausted from the airbearings.

The effective air-spring stiffness applied by the isolator-actuator tothe payload is determined by the gas volume, piston area, gas pressure,and gas thermodynamic constants. This air-spring stiffness incombination with the payload mass properties determines the isolationbreak frequency, i.e. the frequency above which isolation will occur. Ina further embodiment, to achieve a given desired isolation breakfrequency, the invention combines an air tank or accumulator with theair piston volume to achieve a total air volume that achieves therequired air-spring stiffness for the payload mass being supported. Thelarger the added volume of the air tank, the lower the air-springstiffness, and for a given payload mass, the lower the isolation breakfrequency. Because base support vibration loads of frequencies higherthan the break frequency are attenuated and for sufficiently highfrequencies not transferred to the payload, a pneumatic actuator servesinherently as a very effective vibration isolation support device. Theinvention results in a very low vibration frequency isolator and highforce, large stroke, high bandwidth actuator.

BRIEF DESCRIPTION OF THE DRAWINGS

The operation of this invention can be best visualized by reference tothe following drawings described below.

FIG. 1 is a cut away view of a preferred embodiment of a hybridpneumatic-magnetic isolator-actuator.

FIG. 2 a is a cut away view of the coil, coil carrier structure, andmagnetic actuator body in accordance with an embodiment of theinvention.

FIG. 2 b is a cross section view of the coil and magnetic actuator bodyin accordance with an embodiment of the invention.

FIG. 3 a is a cross section view of a hybrid pneumatic-magneticisolator-actuator and air supply system incorporated into a vacuumchamber environment in accordance with an embodiment of the invention.

FIG. 3 b is a close-up cross section view of the air bearing and bellowsdetail of the cross section view shown in FIG. 3 a in accordance with anembodiment of the invention.

FIG. 4 is a control block diagram of a hybrid pneumatic-magneticisolator-actuator in accordance with an embodiment of the invention.

FIG. 5 is a cross section view of a hybrid pneumatic-magneticisolator-actuator incorporating an extension locking device inaccordance with an embodiment of the invention.

FIG. 6 is a view of an embodiment of an extension locking devicesuitable for incorporation into a hybrid pneumatic-magneticisolator-actuator in accordance with an embodiment of the invention.

FIG. 7 is a view of the extension locking device of FIG. 6 shown inposition for extension locking.

DETAILED DESCRIPTION OF THE INVENTION

Described in detail below is a compact and lightweight hybridpneumatic-magnetic isolator-actuator. In the description, for purposesof explanation, numerous specific details are set forth in order toprovide a thorough understanding of the present invention. It will beobvious, however, to one skilled in the art that the present inventionmay be practiced without these specific details. In other instances,well-known structures and devices are shown in simplified form in orderto avoid obscuring the present invention. Additionally, though theembodiments described below refer to air as the gas used, any of variousgases or combination of gases including nitrogen, argon, and heliumcould be used for effecting the isolation, pneumatic force, and coilcooling aspects of the invention.

FIG. 1 shows a preferred embodiment of the invention where a cut awayview of a hybrid pneumatic-magnetic isolator-actuator 100 is depicted.Within the isolator-actuator a single moving carriage is comprised of acoaxially arranged frictionless piston 110, piston connecting rod 150,flange plate 225, lower air bearing 220, journal shaft 130, upper endstop 140, coil 120 and coil carrier structure 125. This carriage isdriven relative to a frame or base support housing comprised primarilyof a cylindrical piston sleeve 210, magnetic actuator body 230, upperair bearing 240, and airtight housing 310. Airtight housing 310 is alsoreferred to as frame 310.

In accordance with the embodiment shown, the carriage is supportedcompletely on an air film so to effect a frictionless isolator-actuator.The piston 110 is of air bearing construction such that the air suppliedbelow the piston for load carrying also supplies air channel ways withinthe piston and gap around the piston to create an air film completelycovering and supporting the piston in the cylindrical piston sleeve 210.The air load effected on the piston 110 is transferred to a connectingrod 150 which in turn is mounted to a flange plate 225. The lower airbearing 220 is concentric to and surrounds the cylindrical piston sleeve210 and is also attached to the flange plate 225. The piston 110 andlower air bearing 220 move together through their common attachment tothe flange plate 225 and together provide for frictionless lateralconstraint and longitudinal motion of the carriage along the cylindricalpiston sleeve 210.

The carriage is further comprised of the coil 120, coil carrierstructure 125 and journal shaft 130. The coil carrier structure 125supports the coil 120 and together they move as part of the carriagethrough the coil carrier structure's attachment to the flange plate 225.The journal shaft 130 is attached at one end to the flange plate 225 andthe other end of journal shaft 130 protrudes through the housing 310.Journal shaft 130 is laterally supported by upper air bearing 240 whichis mounted in the end of housing 310.

In the embodiment of FIG. 1, at each end of isolator-actuator 100 is aspherical joint 320 for providing moment free attachment of theisolator-actuator between the payload and payload supporting structure.Additionally, extension bumper 250 mounted to flange plate 225 andcompression bumper 260 mounted to the end of housing 310 providecushioning of impact of the carriage at the ends of its travel withrespect to the frame. Additionally provided in the embodiment of FIG. 1is a displacement sensor 115 attached between the bottom of the piston110 and housing 310 for providing extension data from theisolator-actuator.

Magnetic actuation capability is combined in parallel with the pneumaticactuation capability. Magnetic force application is achieved and appliedto the carriage via the coil 120 which is attached to the coil carrierstructure 125 and flange plate 225. The magnetic actuator body 230 ismounted within the housing 310 coaxial with the journal shaft 130. Thecoil carrier structure 125 passes through one end of the magneticactuator body 230 and supports the coil 120 within the envelope of themagnetic actuator body. Magnetic forces are developed between the coil120 and magnetic actuator body 230 by controlled application of currentto the coil by flexible wires. These forces are transferred from thecoil through the coil carrier structure and into the journal shaft thusadding these magnetic induced forces with the pneumatic forces appliedvia the piston.

FIG. 2 a shows a view of the coil 120 and coil carrier structure 125 ina cutaway view of the magnetic actuator body 230 in accordance with anembodiment of the invention. FIG. 2 b further shows a cross section viewof the coil 120 and magnetic actuator body 230 in accordance with thisembodiment. In FIG. 2 b, magnetic actuator body 230 is further comprisedof outer iron 231, inner iron 234, and segmented bottom iron 235. Analuminum flux stop 233 is positioned at the axial center of inner iron234. The purpose of the flux stop 233 is to reduce the permeance of thepath taken by magnetic flux produced by the coil, and thus to reduce thecoil inductance. Magnet 232 is attached to the inner wall of the outeriron 231. Coil 120 is effectively surrounded by the magnet 232, outeriron 231, inner iron 234, and segmented bottom iron 235.

The volume of air (or gas) supporting the piston determines theeffective air-spring stiffness applied by the isolator-actuator to thepayload. This air-spring stiffness in combination with the payload massdetermines the isolation break frequency. In an embodiment of theinvention, to achieve a given desired isolation break frequency, theinvention combines an air tank or accumulator with the air piston volumeso to achieve a total air volume that achieves the required air-springstiffness for the payload mass being supported.

FIG. 3 a shows a cross section view of a hybrid pneumatic-magneticisolator-actuator 100 and air supply system incorporated into a vacuumchamber environment in accordance with an embodiment of the invention.Hybrid pneumatic-magnetic isolator-actuator 100 is connected toaccumulator tank 550 by air line 555. The volume of the accumulator tank550 and air line 555 added to that below piston 110 within hybridisolator-actuator 100 determines the air-spring stiffness of the system.The larger the total volume, the lower the air-spring stiffness, and fora given payload mass such as bench 600, the lower the isolation breakfrequency that is achieved. Base support vibration loads of frequencieshigher than the break frequency are attenuated and for sufficiently highfrequencies are not transferred to the payload 600.

A particular advantage of the hybrid pneumatic-magneticisolator-actuator 100 is its suitability for use in vacuum environments.In FIG. 3 a, isolator-actuator 100 is contained within a vacuum box 500.Compressed air for feeding the air bearings is supplied from outsidevacuum box 500 by compressed air line 510. Accumulator 550 suppliescompressed air to the air piston from outside vacuum box 500 by air line555 which utilizes the accumulator port 340 depicted in FIG. 1. Allsupplied compressed air to isolator-actuator 100 is contained withinairtight housing 310 as shown previously in FIG. 1. The suppliedcompressed air is exhausted from isolator-actuator 100 by air exhaustline 520 which carries the air through the wall of the vacuum chamberbox 500 via an airtight feed-through 525.

FIG. 3 b is a close-up cross section view of the upper air bearing andbellows detail of the cross section view shown in FIG. 3 a in accordancewith an embodiment of the invention. Air supplied by compressed air line510 of FIG. 3 a feeds upper air bearing 240 via air bearing feed andreturn fittings 330 shown in FIG. 3 b and FIG. 1. Bellows seal 350 shownin FIG. 1 and in large cross section in FIG. 3 b works in conjunctionwith airtight housing 310 to prevent air escape into vacuum box 500while allowing relatively unimpeded longitudinal motion between thecarriage and frame.

In a preferred embodiment, the construction of hybrid pneumatic-magneticisolator-actuator 100 is such that air supplied to the air bearings 240and 220 is channeled over coil 120 for providing coil cooling.

An important characteristic of the embodiment of the invention shown isthat the construction provides for the parallel pneumatic and magneticforce application to the actuator carriage within a relativelylightweight and compact uniaxial strut. The uniaxial construction avoidsthe bending loads and associated stresses common to nested actuatorsystems such that significantly higher maximum loads can be endured.Aircraft payloads may be supported and constrained against crash loadswithout the need for redundant constraint systems. Further, the controlof the hybrid pneumatic-magnetic isolator-actuator is such that thepneumatic piston 110 provides payload positioning force for both staticloads and low frequency dynamic loads. The air pressure applied to thepiston 110 either directly or by accumulator 550 is controlled by apressure servo-valve which varies the piston pressure in response tomeasured and predicted changes in the required isolator-actuator supportforce or other desired controlled state variable. The invention therebyeffects the capabilities of a frictionless pneumatic actuator as well aspneumatic vibration isolator. The low bandwidth control capabilityinherent in pneumatic actuators is augmented by the high bandwidthmagnetic force application ability built into the isolator-actuator toeffect a medium to high bandwidth hybrid pneumatic-magneticisolator-actuator.

The manner for effecting the hybrid pneumatic-magnetic actuationcapability is depicted in FIG. 4 which shows a control block diagram ofa hybrid pneumatic-magnetic isolator-actuator with integrated control ofthe magnetic force application in accordance with an embodiment of theinvention. In FIG. 4 desired actuator force command voltage 700 is inputto the pressure control loop 800 of the pneumatic pressure system.Servo-valve driver amplifier 810 applies amplified command voltage tothe pressure drive plant 880 comprised of the servo-valve, pressuresupply, and accumulator tank. Plant 880 output air pressure 720 resultsand is applied over piston area 830 to result in piston pneumatic force740. Within pressure control loop 800, pressure 720 resulting from plant880 is measured by pressure transducer/amplifier 820 and the outputpressure voltage signal 730 is subtracted from the force command voltage700. Closed loop pressure control is thereby effected on plant 880 insupplying air pressure over piston area 830 of the pneumatic piston.

As shown further in the control diagram of FIG. 4, the pneumatic systemis then combined with a high bandwidth magnetic drive capability toovercome the relatively low bandwidth performance nature of pneumaticactuators. An example hardware embodiment of this magnetic drivecapability is the integral construction of the coil 120 and magneticactuator body 230 into the hybrid pneumatic-magnetic isolator-actuatorof FIG. 1. The pressure error voltage 735 is fed forward in an open loopsense to the helper gain amplifier 851. In a preferred embodiment of theinvention, helper feed forward loop 850 is constructed such that thehelper gain value of 1.0 applied at helper gain amplifier 851 exactlycompensates for the measured pressure error in the pressure servo loop800. Voltage amplifier 852 and current-drive power amp 853 appropriatelycondition and feed the input error signal 735 to the magnetic actuator854 such that an exact compensating magnetic force 750 is added topneumatic force 740. The resulting output force 900 from the hybridpneumatic-magnetic isolator-actuator therefore has the dynamic bandwidthcapability associated with magnetic actuators, but is of substantiallylarger static and dynamic amplitude capability than that available froma magnetic actuator employed alone.

A preferred embodiment of the invention combines the electronicsnecessary for implementing the control scheme depicted in FIG. 4 locallywith the hybrid pneumatic-magnetic isolator-actuator hardware depictedin FIG. 1. A self-contained unit for providing very low frequencyvibration isolation and high stroke, high force, high bandwidthactuation is obtained.

In a further embodiment of the invention, an extension locking device isadded to the hybrid isolator-actuator for maintaining payload positionduring power loss and/or for locking the isolator-actuator strut duringexcessive load applications. An example is shown in FIG. 5 whereextension locking device 380 is shown built within a hybridpneumatic-magnetic isolator-actuator 90. Device 380 is positioned withinisolator-actuator 90 such that its actuation causes spacer chocks 381 tomove radially inward so to prevent the carriage from motion relative tothe base support housing. When moved radially inward, the chocks occupythe space between the flange plate 225 on the inboard end of the journalshaft 130 and the inboard end of the body of the magnetic actuator 230.When the chocks 381 are so positioned, the carriage of theisolator-actuator 100 is held in its fully retracted position within theisolator-actuator frame 310.

FIG. 6 depicts a detailed isometric view of locking device 380. Chocks381 are arranged around the interior perimeter of frame 310 and aresupported on bellcranks 386. Bellcranks 386 pivot on shafts 387 and arespring-loaded by torsion springs 382 such that the chocks 381 are heldat their radially inward-most position whenever no force is exerted onthe bellcranks 386 by the unlock actuation cylinders 383. Application ofair pressure to unlock actuation cylinders 383 drives bellcranks 386such that chocks 381 move radially outward. With chocks 381 sopositioned, the carriage assembly can move to extend from the frame 310of the device. FIG. 6 depicts unlocking device 380 in its actuated modewherein unlock actuation cylinders 383 are pressurized and chocks 381are positioned radially outward. Upon removal of air pressure to unlockactuation cylinders 383, torsion springs 382 cause chocks 381 to returnto their positions of FIG. 7 where the chocks 381 reside radiallyinward, away from the interior periphery of frame 310.

In the embodiment of FIG. 5, locking device 380 is attached to frame 310and is positioned such that chocks 381 are radially positionable betweenmagnetic actuator body 230 and flange plate 225. Chocks 381 therebymechanically limit the motion between the moving carriage and stationarybase support housing of the isolator-actuator 90. Locking device 380 mayalso be constructed such that chocks 381 clamp on to the base of coilcarrier structure 125 and thereby prevent carriage motion at whateverextension position the isolator-actuator may be in.

In the embodiment of FIGS. 6 and 7, the locking mechanism 380 isunlocked only when air pressure is applied to unlock actuation cylinders383. Loss of air pressure to the main pneumatic actuator piston 110 andthe unlock actuations cylinders 383 for any reason will cause thecarriage of the isolator-actuator 90 to retract into the frame 310 underthe weight of the payload 600 and the locking mechanism 380 will lockthe carriage in this fully compressed position. The locking mechanism380 as integrated into the actuator-isolator 90 in the manner shown inFIG. 5 is therefore fail-safe, a desirable attribute for any airborneequipment.

It is appreciated that the actuation of the locking device may beperformed in various ways in accordance with embodiments of theinvention. Electric motor or electric solenoid drives could be used inplace of the pneumatic lock actuation cylinders. For the pneumatic lockactuation cylinders, various gases could be used in place of air. Theair or gas supply for the lock actuation cylinders can be the same as orindependent of that which drives the piston within the hybridpneumatic-magnetic isolator-actuator.

In a further embodiment the invention serves as a method for supportingand positioning a payload effecting simultaneous low frequency vibrationisolation and large force and stroke position actuation. The methodcomprises the steps of supporting the payload on a gas piston,commanding the gas pressure applied to the gas piston with a pneumaticservo-valve, measuring the error in pressure resulting upon the gaspiston, and applying a magnetic force in parallel to the resulting gaspressure in proportion to the measured pressure error.

In a further embodiment of the method, the step of supporting thepayload may be further comprised of sizing a gas tank and a cylindersupporting the piston to a volume providing a gas-spring stiffness toyield a desired low vibration isolation frequency of the payload. Thestep of supporting the payload on a gas piston may be further comprisedof supporting the payload on a frictionless piston supported on gasbearings. The method may further comprise the steps of completelycontaining the piston and gas bearings within a gas tight housing andfeeding and exhausting the piston and bearings such that the method issuitable for vacuum environment application.

Hence, a hybrid pneumatic-magnetic isolator-actuator is disclosed. Theforegoing descriptions of specific embodiments of the present inventionhave been presented for purposes of illustration and description. Theyare not intended to be exhaustive or to limit the invention to theprecise forms disclosed, and obviously many modifications and variationsare possible in light of the above teaching. The embodiments were chosenand described in order to best explain the principles of the inventionand its practical application, to thereby enable others skilled in theart to best utilize the invention and various embodiments with variousmodifications as are suited to the particular use contemplated. It isintended that the scope of the invention be pre-defined by the claimsappended hereto and their equivalents.

1. A method for supporting and positioning a payload effecting simultaneous vibration isolation and large force and stroke position actuation comprising the steps of: a) supporting the payload on a frictionless gas piston of gas bearing construction, b) commanding gas pressure applied to the frictionless gas piston with a pneumatic servo-valve, c) measuring the error in pressure resulting upon the frictionless gas piston, and d) applying a magnetic force in parallel to the resulting pressure in proportion to the measured pressure error.
 2. The method of claim 1 wherein the step a) of supporting the payload is further comprised of sizing a gas tank and a cylinder supporting the frictionless gas piston to a volume providing a gas-spring stiffness to yield a desired low vibration isolation frequency of the payload.
 3. The method of claim 1 wherein the step d) of applying a magnetic force in parallel is further comprised of applying a magnetic force to a coil attached to the frictionless gas piston.
 4. The method of claim 1 wherein the coil and frictionless gas piston are attached via a common uniaxial carriage.
 5. The method of claim 4 wherein the uniaxial carriage is supported on gas bearings such that the carriage is completely supported laterally on a film of gas.
 6. The method of claim 5 wherein the gas bearings and frictionless gas piston are completely contained within a gas tight housing, and bearing feed pressure is supplied by a pressure line to the gas tight housing, and bearing escape gas is scavenged and drawn off with a gas scavenging line from the housing such that the carriage, bearings and housing effect a gas tight isolator-actuator unit suitable for vacuum environment usage. 